Anderton
Have you tried re-installing from the original CD?
*snert*
To the OP, some more details would be very helpful such as the version you are using (and your version history on the system and in general). Where is the messge comming from (Windows or Sonar or something else)? Are you using CCC? Is this a fresh install or is it an old install that has just decided to throw the error?
and pretty much anything else you think may be relevant.
Also system specs could be relevant and whether you are having any other problems with other programs (or the system is acting weird at all in other ways).
Really though... without more info Craig (and the message) likely gave you pretty much the best course of action. Backup any existing projects to an external drive (using Per Project Audio folders if possible) and/or create a system image then uninstall and reinstall Sonar. Use something like CCleaner or Revo Uninstaller if you cannot access the Sonar uninstaller.
Welcome to the forum.